The candidate says:
"I don't really like that kind of environment"
---and that is not your answer becuase??! haha---
Let's see here, is this the "environment" you are interviewing him for? If so, I think that is a big hint.
I want to start this off by saying you are absolutely right to question his interest level, as a candidate will never tell you on his own, so .......you dig.
To note, With software developers it is always more difficult to judge social psychological cues, since many engineer types are some what socially awkward, not all and this is getting better (I know I know, I'm going to get called out on that one, but as one I think I have some say, ok!, so back the F off!)
What I am trying to say is, for example, if you were hiring someone for a hair stylist position, you would probably get a lot more feed back and impression if the person wasn't interested.
You need to keep in mind the reality is that in this US economy, many people are working jobs that they may not be naturally interested in. (That's why it is called work in the first place but that is another issue.)
So he may be wrestling himself with the same question - It could be very likely that he doesn't know either.
Some things I have seen and done -If you can, during the interview, walk him around the office during busy times, introduce him to different people who he might work with, talk with him, see how he interacts and responds. See if he talks about anything personal you can get hints from - not conclusions - hints. For example, during an interview someone told me they were moving out becuase his girlfriend just moved out here. He made it clear he was going to be in this specific area, so at least I knew his scope was much smaller. Just information gathering as much as possible, put it in a spread sheet if you have to.
Next, what is the impact of you just asking: "What is your interest level in this position?" If not a big impact, just ask. Yes, he may give you a canned answer. BUT if you listen carefully you can look for the intent behind the answer.
In the end, there is always risk. He could be 100% interested today and loose interest in a month, you never know, but try to lower the risk as much as possible by gathering information.